Dendritic Cells (DCs) are the most potent antigen (Ag) presenting cells and play a central role in directing innate and adaptive immune responses. Mature, activated DCs can stimulate antigen (Ag)-specific immunity, whereas immature DCs can generate Ag-specific immune tolerance. Factors that affect DC maturation status can therefore influence the quality of an overall immune response. Immunoglobulin G (IgG) receptors (FcgammaRs) on DCs mediate the uptake of IgG-opsonized Ags. They can further affect DC maturation via intrinsic activating or inhibitory signaling domains. We have identified the activating (CD32a) and inhibitory (CD32b) isoforms of Fc-gammaRII co-expressed on populations of freshly isolated human DCs and on human DCs derived from monocyte precursors in vitro. We have further found that selective targeting of CD32a leads to DC maturation, whereas co-ligation of CD32b limits this process. This suggests a benefit to targeting opsonized Ag to CD32a on DCs to promote DC maturation and ultimately Ag-specific immunity. Conversely, it also implies a rationale for targeting Ag to CD32b on DCs to preserve the immature state of DCs and generate Ag-specific tolerance. These types of studies with human cells have not been possible due to lack of reagents capable of distinguishing CD32a and CD32b on the surface of live cells. Given the availability of a new reagent to distinguish these divergent isoforms, we hypothesize that CD32a and CD32b on human DCs can each mediate the phagocytosis of opsonized Ags, though they will have different effects on Ag processing and DC maturation that will culminate in qualitative differences in T cell responses. We will address these hypotheses in the following testable aims: 1. characterize the expression and modulation of activating and inhibitory Fc-gammaRs on human DCs; 2. study the function of CD32a and CD32b on human DCs and their roles in Ag processing and presentation; and 3. target Ag-containing immune complexes to CD32a and CD32b to generate Ag-specific T cell immunity and tolerance, respectively. These studies will yield important findings relevant to the increasing clinical use of monoclonal antibodies, and to DC-based vaccines. They should also reveal new insights into the pathophysiology of immune complex-mediated autoimmune diseases.